The invention is related to the field of nuclear magnetic resonance (xe2x80x9cNMRxe2x80x9d) sensing methods and measuring techniques. More specifically, the invention is related to making NMR measurements during well logging or during Measurement-While-Drilling (xe2x80x9cMWDxe2x80x9d) within earth formations surrounding a wellbore. The invention also relates to methods for using NMR measurements to determine petrophysical properties of reservoir rocks and properties of fluids in the earth formations surrounding the wellbore.
The description of the background of this invention, and the description of the invention itself are approached in the context of well logging because well logging is a well known application of NMR measurement techniques. It is to be explicitly understood that the invention is not limited to the field of well logging.
An apparatus described in U.S. Pat. No. 4,710,713 issued to Taicher et al is typical of NMR instruments used to measure certain petrophysical properties of earth formations from within a wellbore drilled through the earth formations. NMR well logging instruments such as the one disclosed by Taicher et al typically include a magnet for polarizing nuclei in the earth formations surrounding the wellbore along a static magnetic field, and at least one antenna for transmitting radio frequency (xe2x80x9cRFxe2x80x9d) energy pulses into the formations. The RF pulses reorient the spin axes of certain nuclei in the earth formations in a predetermined direction. As the spin axes precess and reorient themselves into alignment with the static magnetic field, they emit RF energy that can be detected by the antenna. The magnitude of the RF energy emitted by the precessing nuclei, and the rate at which the magnitude changes, are related to certain petrophysical properties of interest in the earth formations.
There are several principal operating parameters in NMR well logging which should be optimized for efficient operation of an NMR well logging instrument. These parameters include the logging speed (speed of motion of the instrument along the wellbore), the average and the peak power supplied to the instrument and transmitted as RF pulses, and the signal-to-noise ratio (xe2x80x9cSNRxe2x80x9d). Other parameters of interest include the vertical resolution of the instrument and the radial depth of investigation of the measurements made by the instrument within the formations surrounding the wellbore. The last two of these parameters are primarily determined by the antenna and magnet configurations of the NMR logging instrument. Improvements to these two parameters are the subject of numerous patents and other publications. Providing more flexibility in the instrument""s peak power requirements, and limitations on the logging speed necessitated by the physics of NMR measurement have been more difficult to overcome.
Proton NMR relaxation time varies for different fluid types in earth formations. In addition, pore-size distributions dictate relaxation rate of wetting-fluid protons, due to the fast surface relaxation and the diffusional mixing of protons near the matrix-fluid interface with those in the middle of pores. Thus, in general, a distribution of NMR relaxation times is often observed for protons of fluids in earth formation. A large number of data points acquired in the same dynamic range is crucial to improve the accuracy and resolution of the relaxation time distribution, particularly because MWD and wireline data are known to be contaminated with high levels of random noise. There is a difference between the longitudinal relaxation time T1 distribution and the apparent transverse relaxation time T2 distribution of fluids filling the pore spaces of the porous medium. The difference is due primarily to the effect of diffusion in the presence of magnetic field gradients. For example, light hydrocarbons and natural gas may have T1 relaxation times of the order of several seconds, while the apparent T2 relaxation times may be only about {fraction (1/100)} that amount because of diffusion when measurements are made in strong gradient magnetic fields. These field gradients can arise from the non-uniformly applied static magnetic field or from the earth formations themselves. The latter gradients are caused by differences in magnetic susceptibility between the solid portion of the earth formation (referred to as the rock xe2x80x9cmatrixxe2x80x9d) and the fluids filling the pore spaces: both the magnitude and direction of these gradients are difficult to predict).
In wireline NMR logging, the types of NMR measurements affects the logging speed. NMR measurements often require the nuclei of the material be polarized by the static magnetic field for more than three times the longest T1 relaxation time of any individual component within the material. This requires very slow logging speeds and, in many circumstances, is unacceptable.
Typical NMR well logging measurements use pulsed NMR techniques in which RF energy is transmitted to the measurement sensitive volume in the form of a series of pulses. The most commonly used pulse sequence for logging application is the Carr-Purcell-Meiboom-Gill (xe2x80x9cCPMGxe2x80x9d) pulse sequence. For well logging applications known in the art, the CPMG pulse sequences are about 0.01 to 1 seconds in total duration, depending on the number of individual pulses and the time span (xe2x80x9cTExe2x80x9d) between the individual RF pulses. Each series of CPMG pulses can be referred to as a xe2x80x9cmeasurement setxe2x80x9d.
The efficiency of NMR logging is affected by the following three aspects. Firstly, the wait time between two acquisition cycles is dictated by the formation rock and fluid properties. Thus, maximizing the number of data and experiments to be acquired within one measurement cycle is desired. Secondly, power transmission duty cycle, defined as the ratio of the RF transmitting time vs. total time, is limited by the instrumentation design and the efficiency of heat dissipation. In some existing NMR logging tools, the duty cycle is as low as 3-4%. When the measurements are limited by duty cycle, the tool can not repeat the experiment as fast as the formation wait time allows. Thus, use of pulses or pulse sequences that minimize the RF power consumption yet provides the same information is desired. Thirdly, the number of repeated measurements depends on the required signal-to-noise ratio. Formation properties, such as porosity, affect the signal strength. The conductivity of formation and/or borehole affects the RF energy transmission efficiency and, consequently, the strength of noise. Different porosity distributions may also require different SNR in order to achieve a desired accuracy of porosity estimates. Specifically, faster relaxing components require higher SNR data compared to slower relaxing components. Thus, the number of experiment repeats is desired to be higher for the portion of the signal that represents fast relaxing protons most.
The CPMG sequence is commonly used for well logging applications because it acquires a series of NMR signal amplitudes of a vital decay range, time-spaced equally, within a single polarization cycle. Although TE is desired to be as short as the instrumentation permits, the short TE is beneficial primarily for resolving fast relaxing components. For slowly relaxing components, the choice of TE must be balanced with power requirements to avoid limiting the number of echos acquired. It is desirable to choose the time series in accordance with the relaxation distribution scale at which one wants to resolve the spectrum, rather than taking the data equally-spaced in time. Although CPMG is efficient in terms of a large number of echos that can be acquired within a single polarization cycle, it is not an efficient way to use available RF energy because the data are acquired equally time-spaced while the relaxation components are logarithmically time-spaced.
For MWD, where high frequency vibrations limit experiment time, the saturation-recovery sequence for T1 measurement (Fukushima, and Roeder, p. 169, Experimental Pulse NMR, Addison-Wesley, 1981; Taicher and Reiderman, ""205; Prammer, et al., SPWLA paper #EEE, Dallas, Tex., Jun. 7, 2000) is preferred to the CPMG sequence as it can be designed to be less sensitive to vibrations by using broadband saturation pulses. Despite being a relatively fast acquisition sequence compared to the inversion-recovery sequence, the saturation-recovery MWD acquisition still takes long compared to a CPMG T2 acquisition and, thus, limits the number of measurements to less than ten in almost all cases. The limited sampling of the T1 recovery limits ones ability to decompose the recovery times into a relaxation spectrum or, even worse, differentiate between slow and fast relaxing components.
Besides being less sensitive to vibration than CPMG, T1 measurements have several other merits for formation evaluation. Firstly, unlike T2, T1 measurements are insensitive to proton self-diffusion in the presence of a magnetic field gradient, which appears as an additional decay mechanism in T2 measurements. Therefore, interpretation of formation and fluid properties from T1 is simpler. On the other hand, comparison of T1 and T2 information from the same formation system makes it possible to distinguish a fluid component that is diffusion dominant, such as hydrocarbon gas.
U.S. Pat. No. 6,049,205 to Taicher et al. (xe2x80x9cTaicher ""205 xe2x80x9d) teaches a method for determining the nuclear magnetic resonance longitudinal relaxation time T1 of a medium. The method is a time-efficient version of the inversion-recovery sequence for T1 measurements when using multiple frequencies to excite protons in mutually non-overlapping sensitive volumes within one polarization cycle. In the method, a number, N, of 180xc2x0 xe2x80x9cinversionxe2x80x9d pulses, one pulse at each of the excitation frequencies, fl through fN, are transmitted first to invert proton spins in each of the corresponding sensitive volumes. There need be negligibly small wait time between these inversion pulses because the corresponding sensitive volumes are non-overlapping. The 180xc2x0 xe2x80x9cinversionxe2x80x9d pulses are then followed by a first (shortest) recovery time R1 after which a first read-out CPMG pulse sequence with a duration Ttr. A second CPMG follows, transmitted with a second frequency. The procedure is then extended to all N frequencies.
Taicher ""205 also teaches the determination of the transverse relaxation time distribution of the medium with an improved signal-to-noise ratio. The medium is polarized along a static magnetic field. A first CPMG echo train is acquired from within a first sensitive volume. The first CPMG train has an inter-echo spacing and a duration long enough to determine the presence of slowly relaxing components in the medium. Then a plurality of additional CPMG echo trains is acquired. Each of the additional echo trains corresponds to a different sensitive volume, and each of the additional CPMG echo trains has an inter-echo spacing and a duration less than the duration and echo spacing of the first CPMG echo train. Different sensitive volumes are measured by transmitting each additional CPMG sequence at a different radio frequency. In the preferred embodiment, the additional echo trains have a duration and inter-echo spacing adapted to determine the presence of components in the formation having a transverse relaxation time less than about 33 milliseconds. The total duration of all the additional echo trains is about equal to the duration of the first echo train. In the preferred embodiment, the total radio frequency power transmitted in the all the additional echo trains is approximately equal to the radio frequency power transmitted in the first echo train.
The present invention includes a pulse sequence scheme to obtain T1 and T2 data that each emphasizes a specific portion of the relaxation time distribution and porosity distribution, and combining T1 and T2 data to obtain the porosity distribution formation rock and fluid properties. The pulse sequence is a rapid method of making nuclear magnetic resonance measurement of a medium, using an NMR tool that has a well-defined static magnetic field gradient, so that the magnetic filed strength varies spatially. The nuclear spins of the medium are magnetically polarized under the static magnetic field. Data are acquired at at least two different radio frequencies from at least two different sensitive volumes of the medium. In a preferred embodiment of the invention, more than two frequencies are used to speed the data acquisition and improve the data quality. For each of the at least two sensitive volumes, an RF pulse sequence is defined, each pulse sequence comprising at least one sub-sequence having a saturation pulse, a recovery pulse that follows the saturation pulse after a defined saturation time, and at least one refocusing pulse. In a preferred embodiment of the invention, a series of refocusing pulses is used so that a series of echoes are acquired from which a T2 distribution, corresponding to either fully or partially polarized signals, can be derived. The wait times for the sub-sequences of the various sensitive volumes are selected from a distribution of values between a minimum time to a maximum time. In a preferred embodiment of the invention, the data acquisitions for T1 and T2 decay data are interleaved among multiple frequencies. Specifically, during the wait time between the saturation pulse and the recovery pulse of the first frequency activation, data acquisition of one or more additional frequencies are started and data acquisition of these frequencies may be completed before or after the data acquisition of the first frequency. The interleaving pattern can be either regular or irregular. The interleaving may be nested. Typically, the minimum and maximum wait times are set at 0.1 ms. and 10 seconds. The distribution of wait times may follow a power law relationship approximately to be in accordance with the desired resolution scale of the T1 and T2 spectrum. The wait times for two sub-sequences at a particular frequency may be the same: in such a case, the corresponding refocusing pulses are phase alternated, making it possible to reduce the effects of ringing.
In a preferred embodiment of the invention, the bandwidth of the saturation pulse or pulses for a sensitive volume is greater than or equal to the bandwidth of the corresponding recovery and refocusing pulses. The frequencyxe2x80x94separation of the RF pulsesxe2x80x94is chosen to be greater than the bandwidth of the RF excitation pulses in the sequence.
In another embodiment of the invention,-echo trains acquired with different wait times can be stacked to improve the signal to noise ratio. The resultant echo train is inverted to obtain a T2 spectrum. Preferably, only the fully polarized T2 components (fast relaxing components) may be analyzed and partially polarized components are discarded. The number of echos (NE) following individual recovery pulse can be the same or different. Even if the individual NE parameters are different, they can still be averaged. The T2 data are used to interpret earth formation and reservoir fluid properties that have characteristically fast relaxation times while the T1 spectrum is used to interpret properties that have characteristically longer relaxation times (e.g., greater than 3 ms). The total porosity is obtained from the echo train acquired after the longest wait time after the saturation pulse. This wait time is sufficiently long to ensure the achievement of full polarization of all relaxation-time components. The total porosity is used as a constraint in the process to combine T1 and T2 spectra.